Vacuum deposition is a common method for coating metals such as aluminum, copper, zinc, and tin onto various substrates of metal, glass, and plastic. The metal is typically vaporized by means of electric resistance heating in a metallic or ceramic container or vessel generally referred to in the art as a “boat” or a metallization boat. The boat is connected to a source of electrical power within an evacuated chamber and heated to a controlled operating temperature sufficient to cause a metal charge placed in contact with the boat to vaporize.
In the vacuum metallization process, the metal melt in a metallization boat is heated to a very high temperature, in many instances to a temperature higher than typically seen in casting operations, 1200° C. and above. This means that the metal melt will be much more aggressive and behave as a corrosive acid, lowering the life of the metallization boats. Additionally, when the operation is carried out with repetitive heating, cooling, and exposure to molten metals during that span, the life of the boats is further reduced.
One important factor to longer boat life is the wettability of the slag build-up in the cavity of the boat. After a few hours in operation, a slag begins to build up around the edge of the metal puddle in the boat. The slag is the by-product of the reaction of the molten metal with the refractory boat, and is typically non-wetting to the metallizing metal. It inhibits the spreading of the metal puddle, thus increases the operating temperature and further reduces the useful life of the boat. Improved wetting of the boat and/or slag surface is believed to be an important factor in increasing the average life of metallization boats.
Another factor impacting boat life is the resistance to corrosion by the molten aluminum. During metallization, the molten aluminum corrodes the surface of the boat, typically resulting in deep grooves running from the anterior to the posterior of the boat and penetrating into the boat's depth. These grooves over time, effect the spreading of the aluminum melt, affecting the deposition uniformity on the metallized substrate. Additionally, the grooves can cause the spattering of the liquid aluminum due to excessive accumulation of the aluminum in the grooves. Aluminum spatter causes holes in the metallized substrate. At the point where there is non-uniformity in the metallized substrate or there are holes in the boats due to metal spatter, the boats are typically replaced. Therefore, minimizing the depth of grooves, or slowing the formation of grooves over the evaporation surface can lead to longer useful boat life.
PCT Publication No. WO 2005/049881A1 discloses a metallization boat with improved wettability, which indirectly can lead to longer boat life. The boat has a plurality of grooves having a depth of 0.03 to 1 mm, a length of more than 1 mm, a width of 0.1 to 1.5 mm, and a distance between the grooves of less than 2.0 mm.
FIG. 23 is a top view showing a metallization boat in the prior art (as discussed in U.S. Pat. No. 6,645,572) having multiple grooves, with a number of grooves with a length-wise orientation as the direction of conduction of the boat, and the rest of the grooves having most of their length running parallel to the direction of conduction of the boat.
There is still a need to further increase the useful life of metallization boats by mitigating the severity and pattern of groove formation over the metallization surface. Applicants have found that increasing the depth, and/or the width, and/or the spacing between the grooves in metallization boats leads to less severe grooving over the metallization surface, and a more uniform grooving pattern, while maintaining the improved wettability, thus extending the useful service life of the boats over the prior art.